Optical fibers have become a promising medium for use in optical communication systems. An exemplary class of fibers is composed of a core portion which is surrounded by a cladding region of refractive index smaller than that of the core. Within this general class of fibers, various designs have been used.
One approach is the use of multimode fibers. The diameters of these fibers are relatively large, typically tens of microns, and can support the propagation of a large number of modes. Each mode carries a fraction of the total light power transmitted through the waveguide. Since each guided mode has a different velocity along the longitudinal axis of the fiber, the light power in each such mode necessarily traverses the fiber in a different time period. Thus, a narrow pulse of light initially introduced in the fiber will emerge as a long series of closely spaced overlapping short pulses. The overall effect is a dispersed emergent pulse.
An approach adopted to reduce modal dispersion in a multimode fiber is to grade the refractive index of the core region. For example, one suggested class of refractive index profiles follows the equation EQU n(r) = n.sub.o [1-2.DELTA. (r/a).sup..alpha. ].sup.1/2
where n.sub.o is the refractive index at the center of the core, a is the radius of the core, .DELTA. is a measure of the refractive index difference between the center of the core and the cladding defined by (n.sub.o -n.sub.c)/n.sub.o, and n.sub.c is the refractive index of the cladding.
The choice of exponent .alpha. determines the particular profile of the class. Typically .alpha. is chosen equal to an .alpha..sub.opt which substantially reduces modal dispersion at a predetermined wavelength. For example a suggested .alpha..sub.opt is ##EQU1## where .lambda. is the wavelength of the guided light, .DELTA.' is the derivative of .DELTA. with respect to wavelength, N.sub.o is equal to n.sub.o -.lambda.n.sub.o ' and n.sub.o ' is the derivative of n.sub.o with respect to wavelength. (See, for example, Olshansky and Keck Appl. Opt., 15 (2) 1483 (1976), U.S. Pat. No. 3,904,268 and Gloge, Kaminow and Presby, Electronics Letts., 11, 19 (1975).) In such graded fibers the refractive index increases from the cladding to the center of the core. Modes whose energy is concentrated nearer to the cladding in a profiled fiber are induced to move faster relative to those modes concentrated closer to the center of the core. The former modes which in the case of a step function refractive index profile would emerge after the lower order modes, emerge essentially together with these modes. Thus total dispersion associated with a source emitting in a very narrow wavelength range, for example 1 Angstrom, is indeed often substantially decreased merely through adjustment of the refractive index profile of the fiber as discussed above. However, dispersion effects resulting from the nonmonochromatic character of the guided light are not usually corrected by such refractive index profile adjustments.
The refractive index produced in a fiber whether through a density or polarizability effect is a function of light wavelength. Thus the delay time of any given mode traversing the fiber will be different for different wavelengths. In a broader spectrum of guided light the shortest wavelength often causes a significantly different traversal time for a given mode than the longest wavelength. The resulting dispersion is a wavelength dependent effect called material dispersion. Additionally, since the refractive index is a function of wavelength, the profile, in most cases, produces the best reduction of modal dispersion for only one wavelength of the guided light. These wavelength dependent effects often become quite significant when a broader spectrum of light, e.g. 200 Angstroms and broader, is guided. Improvement in the information carrying capacity of an optical communication system for a broad spectrum of guided light requires further corrections entailing considerations beyond the overall refractive index profile of the fiber.